In general, a suitable balance weight is provided which is mounted for compensation for mechanical unbalance of a rotary engine in consideration of its mechanism. The rotary engine practically used at present comprises a rotary journal portion or portions provided eccentrically of the output shaft of the engine relative to the axis thereof, on which a rotor is held. Therefore, the rotary engine cannot avoid the mechanical unabalance of the rotatably moving portion. The extent of such mechanical unabalance of the rotary engine is usually several kg/cm which can be compensated by mounting a balance weight or weights on the output shaft of the rotary engine. The balance weights are commonly provided on both sides of the rotary journal portion or portions opposite thereto that is at the angle of 180.degree. relative to the rotor journal portion or portions in a conventional rotary engine. More particularly, in a rotary engine of dual rotor type, one of the rotors has a balance weight mounted on the output shaft of the engine opposite to one of the rotor journal portions that is at the angle of 180.degree. relative thereto, on which one of the rotors is journaled and the other rotor has a balance weight mounted on the output shaft of the engine opposite to the other rotor journal portion that is at the angle of 180.degree. relative thereto. In addition thereto, a flywheel is conventionally mounted on the output shaft of the engine for balancing the variation in rotating power applied to the shaft. It has been proposed that one of the balance weights is mounted on the flywheel for compensation for the unbalance of the rotatably moving parts.
Of late, it has been tried to use a rotary engine of various features as an engine for a bicycle. In the rotary engine for a bicycle, it is necessary that a magneto generator is connected to the output shaft of the engine so that it is driven thereby to provide the power necessary to ignite the engine or to energize lamp load. Conventionally, such magneto generator comprises a stator including igniting coil means to supply igniting power to the engine and lighting or battery charging coil means, and a bowl-like flywheel type magnet rotar rotatably disposed outside of the stator and connected to and driven by the output shaft of the engine. Thus, due to the balance weight or weights mounted on and the flywheel type magneto generator connected to the output shaft of the engine, there has been an increase in the length of the output shaft resulting in large size of the engine. Such increased length of the output shaft is unpreferable in consideration of requirement of lightweight and compactness of the engine for a bicycle.